The special prosecutor investigating Trayvon Martin's death has decided against using a grand jury to determine whether any charges should be brought in connection with the Florida teenager's killing.

A statement released by Angela Corey's office on Monday did not give a reason for the decision.

"From the moment she was assigned, Corey noted she may not need a grand jury," the statement said.

Martin, 17, was shot dead in Sanford, Florida, on 26 February by a self-declared neighbourhood watch captain, George Zimmerman. Police decided not to arrest Zimmerman at the time.

"At this time, the investigation continues and there will be no further comment from this office," the statement said.

Police in the town of Sanford declined to arrest Zimmerman after the shooting, saying they found no evidence to contradict his account that he acted in self-defence. Florida's self-defence law gives leeway to use deadly force and eliminates a person's duty to retreat in the face of danger.

The state attorney previously investigating the shooting, Norm Wolfinger, had said the case would go to a grand jury on Tuesday 10 April. That grand jury would have decided whether to charge Zimmerman but Wolfinger removed himself from the case on 22 March, and he was replaced by Corey.

Corey said her decision to not to use a grand jury should not be considered a factor in determining whether charges will be filed against Zimmerman.

The announcement means the decision on charges now rests solely with Corey, who Associated Press reported has a reputation for not presenting cases before grand juries unless required. Under Florida law, only first-degree murder cases require the use of grand juries.

Also on Monday, Sanford police department was forced to close its doors to the public after dozens of college students converged on the headquarters in attempted act of civil disobedience.

Calling themselves the Dream Defenders, the students represent a coalition from across Florida who began a 40-mile march from Daytona Beach to the department on Friday. The students have called for Zimmerman's arrest, the firing of police chief Bill Lee, the creation of a racially diverse police department review board and a national congressional forum aimed at "eliminating racial inequalities in the US".

The demonstrators arrived shortly after 9am Monday morning with plans to stay until 5pm. Some of them blocked the entrance to the sation. d approximately a half dozen individuals at the entrance to the station.

One demonstrator, Terell BePina, 21, told the Guardian: "We came because we wanted to take action. No more talking. No more saying we're going to do something. We actually came and had an active sit-in.

"We were going to stand and barricade the front doors to the police station until they came out and addressed us."

He said that the students wanted to be arrested. "We wanted them to arrest us. We wanted the public to know, we wanted the community to know, that we're serious."

Norton Bonaparte, the city manager, emailed the Orlando Sentinel, urging the demonstrators to be peaceful. "The city of Sanford hopes the actions of the students will be as peaceful and orderly as the previous rallies and marches have been," Bonaparte wrote. "We want to be accommodating to all our visitors proving they act in a manner that is respectful to the people of the city."